A power supply system has been used, which is structured to supply power to load equipment from a rechargeable secondary battery and to allow charging of the secondary battery even during an operation of the load equipment. Such a power supply system is typically mounted on a hybrid vehicle or an electric vehicle having, as one of vehicle driving power sources, an electric motor driven by a secondary battery.
In the power supply system for a hybrid vehicle, electric power stored in the secondary battery is used as driving electric power of an electric motor for driving the vehicle, and the secondary battery is charged by the power regenerated by the electric motor or by the power generated by a power generator generating electric power as the engine rotates. In such a power supply system, it is necessary to know the charged state (representatively, SOC indicating charge percentage with respect to full-charge) of the secondary battery and to avoid severe state of use that may cause battery deterioration. Specifically, it is necessary to limit output of the electric motor for driving the vehicle that receives an output from the secondary battery and to generate a request for charging the secondary battery or a command to inhibit charging/discharging, based on the estimated value of remaining capacity and, therefore, highly accurate estimation of the remaining capacity is essential.
Conditions of use and output of the secondary battery, however, vary widely as represented by the conditions when it is mounted on a hybrid vehicle, and under the state of use in which charging/discharging is repeatedly executed with relatively high power density, it is generally difficult to estimate the remaining capacity. Thus, improvement in estimation accuracy has been a problem.
In view of the foregoing, in addition to the most basic procedure of successively calculating the amount of change in the remaining capacity based on the integration of input/output current, various procedures for estimating the remaining capacity with high accuracy have been proposed.
By way of example, Japanese Patent Laying-Open No. 2000-306613 (hereinafter referred to as Patent Document 1) discloses a battery state monitoring device in which an internal resistance of a secondary battery is calculated from the relation between battery current (charging/discharging current) and battery voltage (voltage across terminals) measured during use of the battery, an open circuit voltage (OCV) is calculated using the battery current/voltage and the calculated internal resistance, and the battery capacity (SOC) is estimated from the open circuit voltage. Further, in the battery state-monitoring device, if the battery current goes out of or deviates from a prescribed range, the battery capacity (SOC) is estimated by adding, to the battery capacity (SOC) immediately before the deviation, integrated value of battery current after the deviation. Specifically, according to Patent Document 1, the method of estimation is changed dependent on the range of battery current, to improve estimation accuracy.
Similarly, Japanese Patent Laying-Open No. 2000-150003 (hereinafter referred to as Patent Document 2) discloses a charged amount calculating device supporting an SOC calculating procedure based on battery current integration and other SOC calculating procedures. In the charged amount calculating device, the SOC calculating procedure to be used is determined based on a difference between the SOC calculating procedure based on battery current integration and other procedures.
In the battery state monitoring device disclosed in Patent Document 1, the internal resistance that successively changes along with charging/discharging is calculated using inclinations of a plurality of sets of battery voltage and battery current measured during charging/discharging.
It has been known, however, that the change in battery voltage (voltage across terminals) of the secondary battery during charging/discharging delays from the change in battery current. Specifically, when a constant battery current starts to flow or when the battery current stops, the battery voltage is not in a stable state but fluctuates until after a prescribed time period referred to as relaxation time. Therefore, if the battery voltage is measured before the lapse of relaxation time, accurate output voltage of the secondary battery cannot be measured.
Further, the range in which linearity in accordance with the internal resistance appears between the battery current and the battery voltage also changes dependent on the temperature range of the secondary battery. By way of example, in a lithium ion secondary battery, linearity between the battery current and the battery voltage hardly appears at a low temperature range, because of decrease in ion-exchange current density at an electrode surface.
In this connection, though Patent Document 1 discloses that the procedure of estimation based on the calculation of internal resistance and the procedure of estimation based on battery current integration are switched in accordance with the range of the battery current, SOC estimation focused on the temperature condition of the secondary battery or the duration of battery current is not performed. Therefore, it is possible that estimation of open circuit voltage and hence, estimation of remaining capacity (SOC) of the secondary battery involves an error derived from an error in estimating the internal resistance.
Further, in the charged amount calculating device for a hybrid vehicle disclosed in Patent Document 2, switching among a plurality of SOC calculating procedures is determined based on a battery current range and a difference between the SOC value calculated based on battery current integration and the SOC value calculated by other procedure. Therefore, though the SOC calculating procedure based on battery current integration can be switched to other SOC calculating procedure immediately when the battery current becomes low, after a large current flows at a low temperature in which internal resistance is high, there is undeniably a possibility of large error in SOC estimation because of the error in estimating the internal resistance, from the same reason as described with respect to Patent Document 1. Further, the temperature condition of secondary battery and the duration of battery current related to SOC estimation are not sufficiently considered, either. Therefore, the estimation of remaining capacity (SOC) may possibly involve an error.